Conventionally, circuit assemblies that include a composite arrangement of a printed circuit board serving as a control circuit and a busbar circuit unit that is constituted by a plurality of busbars and serves as a large current circuit are known as a circuit assembly that is to be accommodated in an electrical junction box of an automobile. Particularly in recent years, a circuit assembly, as disclosed in JP 2003-164039A, that has a structure in which busbars constituting a busbar circuit unit are directly fixed to a surface of a printed circuit board via an adhesive layer has been proposed, in order to achieve downsizing and space-saving of the electrical junction box.
Terminal sections of electrical components such as a relay and a switch are connected to and mounted on printed wirings of the printed circuit board and the busbars of the busbar circuit unit of such a circuit assembly by soldering such as reflow soldering.
However, the circuit assembly having the conventional structure has the problem that in the process of mounting the electrical components by soldering, adhesion between the busbars and the printed circuit board with the adhesive layer cannot be maintained. Specifically, in recent soldering processes using lead-free solder, the heating temperature in the reflow furnace at the time of soldering needs to be higher than that in the case of conventional eutectic soldering, and may be higher than the melting temperature of plated layers such as tin plating applied to the surface of the busbars. In this case, there is the risk that due to the melting of the plated layer, the adhesion between the adhesive layer and the busbars is not maintained and the busbars are removed or separated from the printed circuit board.
Meanwhile, JP 2007-306672A proposes that plated layers of busbars are made of nickel that has the melting temperature higher than the heating temperature of a reflow furnace. Although this measure indeed solves the problem that the plated layers of the busbars are melted due to the heating temperature at the time of soldering and the adhesion between the plated layers and the adhesive layer is damaged, the nickel-plated terminal sections of the busbars inevitably have a high hardness. Accordingly, if a plated layer of a counterpart terminal that is connected to the nickel-plated terminal section is plated with, for example, tin, which has a lower hardness than nickel, there will be the risk that a new problem may be caused that due to sliding of the contact surfaces of both terminals with respect to each other, the plated layer having a lower hardness wears and the contact resistance increases.